Vienna, Austria - A 19-year-old suspected Islamic State sympathizer planned a suicide attack designed to kill many people at a Taylor Swift concert in Vienna, Austria's intelligence agency said on Thursday.

Austrian authorities detained two suspects Wednesday for allegedly plotting to attack Swift's Vienna shows, which organizers canceled at the last minute.

The main suspect had confessed, saying he "intended to carry out an attack using explosives and knives," domestic intelligence agency (DSN) head Omar Haijawi-Pirchner told a news conference.

"His aim was to kill himself and a large number of people during the concert... either today or tomorrow," he added. The concerts were supposed to run from Thursday to Saturday.

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The second suspect, a 17-year-old, was employed at a facility management company which would have "provided services" at the Ernst Happel Stadium where Swift was to perform, said Haijawi-Pirchner.

The younger suspect, who has so far refused to talk to authorities, was "in the area" of the stadium where he was detained, said Haijawi-Pirchner.

According to Interior Minister Gerhard Karner, a "tragedy was averted". "The situation was very serious," he added. Explosives and detonators were found in a search of the main suspect's apartment, authorities said.

Austria's top security chief Franz Ruf told reporters the two suspects had recently made changes in their private lives,

The main suspect, an Austrian with Northern Macedonian roots, had changed "his appearance and adapted it to Islamic State propaganda," while the second had broken up with his girlfriend, he said.

Ruf earlier confirmed that authorities had received information "from foreign partners" which led to the arrests, but he declined to specify which.

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Police had promised to ramp up security for the concerts while having minimized any concrete danger, but organizers still cancelled Swift's shows. About 65,000 people were expected at each show.

Swift did not immediately comment on the decision to cancel the Vienna shows, but said she was "completely in shock" after three girls were killed in a mass stabbing in England at a dance class inspired by the pop star in England.

According to Ruf, police "did everything humanly possible to ensure" that the Vienna concerts "could go ahead," but the cancellation decision was taken by the organizers.